Japanese Patent Laying-Open No. 5-152638 (Patent Document 1) describes that a piezoelectric film having a piezoelectric constant d14 is obtained by drawing a formed object of polylactic acid, for example. Specifically, as shown in FIG. 19, it is assumed that a piezoelectric film 11 composed of polylactic acid is provided with electrodes (not shown) respectively on its upper and lower surfaces in the figure, and has been drawn in an axial direction “3”. When an electric field is applied in an axial direction “1”, which is a direction normal to the electrode surfaces, shear strain is caused in an axial direction “4”, which is a direction of rotation about axial direction “1”. Such a piezoelectric property is generally referred to as a “shear piezoelectric property”.
When the piezoelectric film composed of such polylactic acid is formed into a unimorph or bimorph structure, a piezoelectric device, such as a piezoelectric vibrator, can be obtained which is used for a speaker, a microphone, or the like. Patent Document 1 describes such general application but provides no specific description regarding its formation method, just reciting “in the case where the formed object thus drawn is a film, it is cut into an appropriate size to serve as a product” (paragraph[0018]).
PTD 1: Japanese Patent Laying-Open No. 5-152638